The present invention relates to the field of surface mounted ("leadless") electronic components and more particularly to the mounting pad configurations of leadless components and the surfaces to which they are to be mounted.
A conventional surface mounted chip carrier, also known as a leadless chip carrier (LCC) has a plurality of electrical contact pads distributed along the periphery of its mounting surface. Usually, the space between the pads is equal to the pad width. A surface mounted component is mounted on a printed circuit substrate by solder bonding of the electrical contact pads of the LCC to an aligned, matching, set of electrical contact pads on the printed circuit substrate. This printed circuit substrate may be a ceramic substrate such as alumina, a fiber glass printed circuit board, or a porcelain coated metal printed circuit board.
Solder attachment of surface mounted components is normally done by coating the contact pads of the component and the printed circuit substrate with solder, placing the chip carrier on the substrate in its intended position and heating the carrier and substrate to melt the solder. This heating step is referred to as reflowing the solder. The composite structure is then allowed to cool to solidify the solder. While the solder is in a molten state, its surface tension provides a force which tends to shift a slightly out-of-alignment chip carrier so that its contact pads are in perfect alignment with the contact pads on the printed circuit substrate.
Printed substrates with surface mounted components mounted on both major surfaces of the substrate are now in use. These substrates are produced by mounting components on a first major surface of the printed circuit substrate in the just-described manner. Once the solder has solidified the substrate is inverted. The components to be mounted on the second major surface are placed on the substrate in proper alignment with contact pads on that surface of the substrate. The substrate and the components on both substrate surfaces are then heated to reflow the solder which will bond components to the second (now upper) surface. During this reflow step the solder bonding the components to the first side of the board also reflows. The surface mounted chip carriers on the first, now lower, side of the substrate are retained on the substrate by the surface tension of the solder between the contact pads of the carrier and the substrate. This is because the combined surface tension of the solder on all of the contact pads of a carrier exceeds the force of gravity on the package.
As circuits have become more complicated, thereby requiring more electrical leads, the chip carrier art has moved in the direction of providing larger packages having narrower electrical contact pads in order to fit more leads on a package.
Several potential problems are presented by the use of larger chip carriers. First, as the size of chip carriers increases, their mass also increases. In consequence, there is a concern for the ability of the electrical contact pads to provide sufficient solder surface tension to hold components on the lower side of the substrate during solder reflow of components on the upper side of the printed circuit substrate. Second, achieving proper alignment between the carrier contact pads and the substrate contact pads becomes more difficult because of the larger size of the carrier and the smaller size of the contact pads.